When there are two (or more) with equal rank, they go by date of promotion. The one with the earliest date assumes command.
At least that is the way I understand it.
https://www.youtube.com/watch?v=al7ont2noYA
That’s just a few months ago. He looks fine. Delivers what the teleprompter says, and changes his tone of voice to emphasize what he wants.
The never Trumper swamp filth on FR are desperate for Trump to have a stronger opponent.